Microchannel plates

ABSTRACT

A tapered, spherically slumped, microchannel plate.

This application is the national phase of international applicationPCT/GB95/01711, filed Jul. 19, 1995 which designated the U.S.

This invention relates to microchannel plates (MCPs).

Recently, it has been demonstrated by Fraser et al (Fraser, G W,Brunton, A N, Lees, J E and Emberson, D L Nucl. Instr. and Meth. A 1993,334, 579) that spherically slumped MCPs may be used as focussing andcollimating X-ray optics. Spherically slumped MCPs have been shown toobey the well known "thin lens" equation: ##EQU1## where 1_(s) and 1_(f)are the source and focal distances respectively, f is the focal lengthof the MCP and R is the radius of curvature of the MCP (taken to bepositive when the source is positioned on the concave side of the MCP).If a point source of X-rays is positioned at the MCP focus (on theconcave side of the MCP at a distance R/2) then 1_(s) =R/2 and 1_(f) =∞.In other words, X-rays passing through the MCP should, ideally, emergefrom the MCP, after grazing incidence reflection, as a collimated beamtravelling parallel to the optical axis defined by the line joining thepoint source and the centre of the MCP.

In Fraser et al a spherically slumped MCP with constant cross-sectionalthickness was employed. In FIG. 1, a MCP 10 of this type, withcross-sectional thickness L, is illustrated. In the coordinate systemused the x axis is defined as the optical axis. It was noted in Fraseret al that whilst a substantially parallel beam of X-rays 12 could begenerated from a point source 14 at a distance R/2 from the MCP, theintensity distribution of this beam in a plane perpendicular to theoptical axis was highly non-uniform. FIG. 2a shows such a non-uniformtwo dimensional X-ray image 20 generated with radiation of wavelength44.7 Å. FIG. 2b shows the X-ray intensity distribution 22 in an axialcut through image 20 with a slice width of 6 mm. The non-uniformintensity distribution is due to the presence in the beam of a mixtureof those rays experiencing one grazing incidence reflection in thechannels, and those rays which pass through the channels without beingreflected.

The non-uniformity of the X-ray beam is unfortunate, since a parallelX-ray beam of uniform intensity (a "flat field") is highly desirable ina number of applications such as X-ray lithography. Conventionally, aparallel or quasi-parallel beam can only be produced by maximising theseparation between the source and the plane of interest, with anattendant drop in the intensity of the X-ray beam.

The present invention is based upon a novel MCP configuration whichgreatly improves the uniformity of the parallel X-ray beam.

Particles which have equivalent de Broglie wavelengths to X-rays, suchas thermal neutrons, are within the scope of the invention.

According to the present invention there is provided a taperedmicrochannel plate.

The MCP may be spherically slumped and may be used to collect X-raysemanating from a point source and generate a collimated beam thereof.The length of the capillary channels may vary as a function of thedistance of the channels from the optical axis so as to ensure that theprobability of X-rays reflecting only once from the interior of thechannel is high. The capillary channels may be circular in cross-sectionand the length of the channels (and hence the cross-sectional thicknessof the tapered MCP) may be substantially described by the equation:##EQU2## where D is the diameter of the channels, R is the radius of thecurvature of the MCP, y is the perpendicular distance of the channelfrom the optical axis and L(y) is the length of a channel at y.

The tapered MCP may be fabricated by grinding a MCP with a numericallycontrolled grinding machine.

A tapered microchannel plate according to the present invention will nowbe described with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a prior art spherically slumped MCP;

FIG. 2 shows a prior art X-ray image and an axial cut through saidimage;

FIG. 3 is a cross-sectional view of a tapered MCP;

FIG. 4 is an image of singly reflected X-rays;

FIG. 5 is an image of unreflected X-rays;

FIG. 6 is an image of doubly reflected X-rays;

FIG. 7 is an image of triply reflected X-rays;

FIG. 8 is a full X-ray image;

FIG. 9 is an axial cut through the full X-ray image;

FIG. 10 is a MCP thickness profile;

FIG. 11 is an axial cut through the X-ray image generated with atruncated MCP; and

FIG. 12 shows the results of a Hartmann test.

In FIG. 3 is shown in cross-section a tapered, spherically slumped,microchannel plate 30 comprising cylindrical channels 32 of diameter D.A source of X-rays 34, such as an electron bombardment source, ispositioned a distance R/2 from the concave side 36 of the MCP on theoptical axis 38. With such an optical arrangement, those X-rays 39 thatemanate from the point source, enter a channel and experience onegrazing incidence reflection in the channel will emerge on the convexside of the MCP as a collimated beam travelling parallel to the opticalaxis.

With prior art spherically slumped MCPs of constant cross-sectionalthickness there is, in addition to singularly reflected rays, a smallproportion of rays which undergo multiple reflection and a significantproportion which pass directly through the channels without reflection.It is this mixture of singly reflected and unreflected rays which givesrise to the highly non-uniform intensity distribution of the outputtedX-ray beam generated by prior art spherically slumped MCPs.

In the present invention the length L(y) of a capillary varies as afunction of the distance y of the capillary from the optical axis.Subject to certain practical constraints, which are outlined below, thevariation of capillary thickness ensures that the probability is highthat X-rays reflect only once from the interior of the channels.

For a spherically slumped MCP with cylindrical channels of diameter D,it is easily shown that the channel length L(y) which ensures that at agiven value of y every meridional ray entering the channel is reflectedonce is given by equation (2), viz: ##EQU3##

The function L(y) corresponds to a thickness profile for an X-ray beamgenerating MCP. Monte Carlo ray trace computer simulations have beenperformed on a Silicon Graphics Challenge XL mainframe to investigatethe two dimensional beam uniformity of such a device. The Monte Carlocode simulates the ray paths of unpolarised X-rays of wavelength 44.7 Å(equivalent energy 0.28 KeV, corresponding to C K radiation) emanatingfrom a point source placed 0.7 m from the concave side of a 40 mmdiameter spherically slumped MCP with a radius of curvature R of 1.4 mand a channel diameter D of 10 μm. In order to account for reflectionsthe code calculates the smooth-surface (Fresnel) reflectivity forunpolarised 44.7 Å radiation.

In FIG. 4 is shown the two dimensional X-ray image 40 calculated by thecode for X-rays experiencing one reflection in the channels. The shadingbar 45 indicates the relative intensity of the image. The imagesdisplayed in FIGS. 4 to 8 are all calculated for a distance 1.0 m fromthe convex side of the MCP and represent the beam intensity profile inthe plane perpendicular to the axis of beam propagation.

In FIGS. 5, 6 and 7 the images 50, 60 and 70 are shown representingX-rays undergoing no reflections, two reflections and three reflections,respectively.

FIG. 5 demonstrates that the MCP configuration all but eliminates X-raysthat pass, unreflected, straight through the MCP channels.

In FIG. 8 is shown the full image 80, obtained by summation of theimages of FIGS. 4 to 7. The full image exhibits excellent beamuniformity, the bulk of the overall beam intensity being contributed bythose X-rays which have undergone single reflection. The uniformity ofthe beam is further demonstrated in FIG. 9 which shows an axial cut 90through the image of FIG. 8.

Modelling of the X-ray beam at distances 0.5 and 1.5 m from the MCPindicates that the beam is substantially parallel. The slight divergenceof the beam is due to contributions from non-meriodional and doublyreflected rays.

A practical MCP may possess a thickness profile based substantially onequation (2), but some truncation of the MCP thickness around the platecentre is required, since from equation (2) as y→O, L(y)→∞. Thetruncation will produce a drop in intensity in the centre of the X-rayimage, but this intensity "hole" need not be a large one. For instance,FIG. 10 shows a truncated MCP thickness profile 100 for a MCP which,apart from the truncation, is of identical design to the MCP modelled inthe Monte Carlo simulation described above. A separate Monte Carlosimulation was run, incorporating the truncated MCP design of FIG. 10,again for the instance where 44.7 Å X-rays emanate from a point sourceplaced 0.7 m from the concave side of the MCP. FIG. 11 shows an axialcut through the resulting X-ray image. The beam intensity distributionconfirms that the central intensity drop is an extremely minor one.Physically, this is because, whilst the finite MCP thickness around theplate centre reduces the proportion of single reflection events, thereis a partially compensating increase in the proportion of X-rays passingthrough channels near to the plate centre without undergoing reflection.Tradeoff studies indicate that uniformity is best for long wavelengths(λ>40 A) and large (R>1.0 M) radii of curvature (corresponding tosource-optic separations >50 cm).

It should be noted that the required thickness profile need not begenerated by symmetric shaping of both the inner and the outer MCP facesas depicted in FIG. 10. Rather, one face may be shaped to achieve therequired thickness profile whilst the other face retains its sphericaltopography.

The divergence of the beam has also been examined by using the MonteCarlo code to calculate the results of a "Hartmann test" in which a maskwith a regular array of pinholes is introduced between the optic and thedetector. The diameter and shape of the images of the pinholes providesan indication of the so-called local divergence, whilst the position ofthe image centroids in relation to the centres of the holes indicatesthe global divergence. A local divergence of less than 5 mrad and aglobal divergence of less than 5-20 mrad have been specified asacceptable figures.

The calculations have simulated the effect of a mask having two parallelrows of 600 μm diameter holes, one row starting on the optical axis(which contains the MCP centre) and extending (in the planeperpendicular to the optical axis) to a position parallel with an edgeof the MCP with a pitch of 2 mm, and the second row being of similardescription but translated 2 mm to one side of the first row. Theposition of the detector from the MCP is taken to be 0.5 m and theposition of the mask is equidistant from the MCP and the detector (i.e.0.25 m from each). FIG. 12 shows the simulated Hartmann image 120 for aMCP of diameter. 36 mm and radius of curvature R of 1.0 m, havingcylindrical capillaries of 12.5 μm diameter (D). Unpolarised X-rays ofwavelength 44.7 Å are again used in the simulation, and the thickness ofthe MCP is truncated by setting a maximum thickness Lmax of 10 mm. Thethickness profile of the MCP is calculated using the equation ##EQU4## arelationship which is obtained by applying the small angle approximationsin θ˜tan θ˜θ to equation (2). For both sets of MCP dimensions describedpreviously (in which R>>r) there is negligible difference between thethickness profile of equation (2) and the simplified profile ##EQU5##

Analysis of the results displayed in FIG. 12 indicates that the greatestlocal and global divergences are both ˜2 mrad, confirming the estimatesof parallelism described above by running simulations at variableMCP-detector separations.

A spherically slumped MCP of constant cross-sectional thickness may beground to the desired tapered shape by a numerically controlled grindingmachine.

We claim:
 1. A concavo-convex spherically slumped microchannel plate,tapered so that the length of capillary channels at the edge of theplate is less than the length of capillary channels at the centre of theplate.
 2. A microchannel plate according to claim 1, wherein themicrochannel plate is used to collect X-rays emanating from a pointsource and generate a uniform collimated beam thereof, and wherein thelength of the capillary channels varies as a function of the distance ofsaid channels from the optical axis.
 3. A microchannel plate accordingto claim 2, wherein the capillary channels are of circularcross-section.
 4. A microchannel plate according to claim 3, wherein thelength of said channels is substantially described by the equation:##EQU6## where D is the diameter of the channels, R is the radius ofcurvature of the microchannel plate, y is the distance from the opticalaxis and L(y) is the length of a channel at y.